1. Field of the Invention
The present invention relates to a film manufacturing apparatus using an inflation (or air blowing) method, and in particular, the present invention relates to a film manufacturing apparatus in which the size of a bubble is detected by an ultrasonic sensor to control the amount of air in the bubble so as to maintain the bubble at a constant size in an inflation process.
2. Description of the Related Art
In a film manufacturing apparatus using an inflation (air blowing) method, a molten plastic is extruded from a circular die in the annular form. The annular plastic is cooled and becomes a tubular plastic film having a predetermined diameter. The tubular plastic film is then folded into two flat sheets which are wound into a roll. A photoelectric sensor or an ultrasonic sensor is used for controlling the diameter of the tubular plastic film or the width of the folded sheets.
For example, Japanese Examined Patent Publication (Kokoku) No. 59-40468 and Japanese Unexamined Patent publication (Kokai) No. 58-78725 disclose an example using a photoelectric sensor comprising a light emitting element and a light receiving element which are arranged on a line tangential to a tubular plastic film. A frost line appears between the annular molten plastic and the tubular plastic film, and the photoelectric sensor is arranged to detect the size of the tubular plastic film having a predetermined size. The light from the light emitting element is blocked by the tubular plastic film when the size of the tubular plastic film becomes greater than the predetermined size, and the light passes from the light emitting element to the light receiving element when the size of the tubular plastic film becomes smaller than the predetermined size, whereby the size of the tubular plastic film can be detected by the photoelectric sensor and thus the size of the tubular plastic film can be controlled in response to the output from the photoelectric sensor.
Also, Japanese Unexamined Patent Publication (Kokai) No. 61-256212 discloses an example using a plurality of ultrasonic sensors. Each of the ultrasonic sensors can detect a distance between the tubular plastic film and the ultrasonic sensor by transmitting an ultrasonic wave toward the tubular plastic film and receiving an ultrasonic wave reflected at the tubular plastic film. The size of the tubular plastic film can be calculated from the detected distance. In this prior art, four ultrasonic sensors are equidistantly arranged around the tubular plastic film so that the size of the tubular plastic film can be calculated from an average of outputs of four ultrasonic sensors to control the size of the tubular plastic film.
In the film manufacturing apparatus utilizing an inflation method, the annular molten plastic extruded from the circular die is cooled by an air cooling means or a water cooling means and the cooled annular molten plastic becomes a tubular plastic film. The air cooling means merely blows a cooling air to the annular tubular plastic and does not restrict or control the size and the position of the annular molten plastic. Therefore, the size and the position of the tubular plastic film changes, so the above described Japanese Unexamined Patent Publication (Kokai) No. 61-256212 uses four ultrasonic sensors which are equidistantly arranged around the tubular plastic film so that an average of the outputs of the four ultrasonic sensors can be obtained. Usually, when the air cooling means is used, the bubble, which is an enlarged spherical part of the annular molten plastic, is not formed adjacent to the circular die.
The water cooling means (including the case where an auxiliary air cooling means is used with the water cooling means) comprises a cooling ring containing cooling water. The inner periphery of the cooling ring substantially contacts the annular molten plastic and controls the size and the position of the annular molten plastic. Therefore, the annular molten plastic is cooled and solidified in this condition and becomes the tubular plastic film below the cooling ring. The size of the tubular plastic film is thus controlled by the water cooling ring. Therefore, it is not necessary to detect the size of the tubular plastic film in this case.
When the water cooling means is used and sufficient air is blown in the annular molten plastic, a bubble, which is an enlarged spherical part of the annular molten plastic, is formed between the circular die and the water cooling ring. In this case, it is desirable to control the size of the bubble. For this purpose, the inventors of this application have used a photoelectric sensor comprising a light emitting element and a light receiving element to detect the size of the bubble. The light emitting element and the light receiving element are arranged on a line tangential to a bubble having a predetermined size. The light from the light emitting element is blocked when the size of the bubble becomes greater than the predetermined size, and the light passes from the light emitting element to the light receiving element when the size of the bubble becomes smaller than the predetermined size, whereby the size of the bubble can be detected.
Problems of this arrangement are that an operator must have experience and skill to set the light emitting element and the light receiving element on a line tangential to a bubble having a predetermined size, and yet a considerable time is necessary for this work. It is necessary to change the position of the light emitting element and the light receiving element whenever the product to be manufactured changes, and the position of the light emitting element and the light receiving element must be adjusted depending on a film forming speed, a kind of plastic, the thickness of the resultant film, and the temperature of the molten plastic. A problem is that the position of the light emitting element and the light receiving element is not logically decided but must be decided from experience. Especially, the problem is serious when a bubble having a larger diameter (for example, larger than 600 mm) is to be formed, or when a plurality of kinds of products must be manufactured.
As described above, a plurality of ultrasonic sensors are used for detecting the size of a tubular plastic film after an annular molten plastic is cooled and solidified. However, there is no example of using an ultrasonic sensor for detecting the size of a bubble of an annular molten plastic which varies in size or vibrates from time to time. Also, there is no example of using an ultrasonic sensor for detecting the size of an annular molten plastic in the case where a bubble is not formed but a frost line appears.